Electron beam imaging systems are typically used for imaging integrated circuits. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
The integrated circuit is impinged with a beam of electrons, and in turn electrons are irradiated from the integrated circuit. The irradiated electrons are collected at the detector where they are converted to an electrical signal. This electrical signal is amplified by an electrical circuitry and the analog signal that is output is converted (or in other words digitized) to a digital signal using an analog-to-digital converter. The relative difference in electron yield as measured from different locations of the sample produces a contrast in the output of the analog signal.
If the range of the output analog signal is matched appropriately to the input range of the analog to digital converter, then the output of the analog to digital converter is a digital stream that shows the contrast at different locations of the sample, which is used to generate a digital image of the sample, also referred to as the electron beam image. In general, the objective is to resolve the contrast in the sample as clearly as possible. For this purpose, the analog amplifier typically has a variable gain and an offset voltage such that the anticipated range of output of the amplifier circuitry can be set to match the input range of the analog to digital converter to the fullest extent, and thereby enable the maximum contrast in the digital image without over-saturation of the digital image. The numeric value of any given pixel in the digital image is generally referred to as the gray level of that pixel.
When certain structures of an integrated circuit are impinged with an incoming electron beam, they produce an electron signal that is indicative of certain electrical and process parameters of interest in the semiconductor fabrication process. Hence, the gray level in the digital image can be used for monitoring these parameters of the fabrication process. If the mapping between the gray level and the parameter of interest is known by modeling techniques or otherwise, then the gray level can also be used to quantitatively extract the value of the parameter of interest. Alternately, one may be interested in only a relative or approximate comparison between one or more of the parameters at measured at different locations on the substrate, or between different substrates for process-control purposes. In this case, comparing the relative intensity of the gray level at these different locations will suffice. One may also choose to bin across different values of the process parameter by binning the gray levels into multiple bins.
Techniques for monitoring electrical and process parameters on a substrate in a semiconductor process line are useful to control these parameters in the line. The method of using the gray levels in the electron image to monitor these parameters is especially useful, since electron beams can be used to probe very small areas of the integrated circuit without using an electrical probe that contacts the substrate. This has an advantage in that the measurement is non-destructive (since it is contact-less) and that the monitoring can be done early in the line before electrical structures are completely formed (since it requires no probe).
Monitoring and controlling semiconductor process parameters using such an electron beam imaging tool typically requires long-term stability in the relationship between the process parameter of interest on the substrate and the measured gray level in the electron beam image. Unfortunately, such long-term stability is often difficult to achieve.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.